Electron beam ion source with integral low-temperature vaporizer

ABSTRACT

An ion source for ion implantation system and a method of ion implantation employs a controlled broad, directional electron beam to ionize process gas or vapor, such as decaborane, within an ionization volume by primary electron impact, in CMOS manufacturing and the like. Isolation of the electron gun for producing the energetic electron beam and of the beam dump to which the energetic beam is directed, as well as use of the thermally conductive members for cooling the ionization chamber and the vaporizer, enable use with large molecular species such as decaborane, and other materials which are unstable with temperature. Electron optics systems, facilitate focusing of electrons from an emitting surface to effectively ionize a desired volume of the gas or vapor that is located adjacent the extraction aperture. The components enable retrofit into ion implanters that have used other types of ion sources. Demountable vaporizers, and numerous other important features, realize economies in construction and operation. Achievement of production-worthy operation in respect of very shallow implants is realized.

CROSS REFERENCE TO RELATED ART

This application is a continuation of U.S. patent application Ser. No.09/736,097 filed on Dec. 13, 2002, now U.S. Pat. No. 6,452,338 whichclaims the benefit of U.S. Provisional Patent Application No. 60/170,473filed on Dec. 13, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the design and operation of an ionsource for use in the ion implantation of semiconductors, and for themodification of the surfaces of materials. The ion source can beretrofitted into the exiting fleet of ion implanters currently used inthe manufacture of semiconductor devices, particularly those used inComplementary Metal-Oxide Semiconductor (CMOS) manufacturing. The ionsource is specifically designed to accomodate the use of new solid feedmaterials such as decaborane (B₁₀H₁₄) and Trimethyl Indium (TMI), whichvaporize at sufficently low temperatures that currently available ionimplant ion sources cannot use them. Indeed, the currently available ionsources result in disassociation of decaborane when that material isintroduced into them. The ion source has an integral low-temperaturevaporize, and a means of introducing the vaporized feed material into anionization chamber which is also temperature controlled by thevaporizer. The feed material is ionized be a variable energy, variablecurrent, wide-area electron beam which passes through the ionizationchamber, but is largely preventable from interacting with the chamberwalls. The ion source also incorporates a gas feed for introducinggaseous materials from pressurized gas cylinders.

2. Description of Prior Art

Ion implantation is a key enabling technology in the manufacture ofintegrated circuits (IC's). In the manufacture of logic and memory IC's,ions are implanted into silicon or GaAs wafers to form the transistorjunctions, and to dope the well regions of the pn junctions. By varyingthe energy of the ions, their implantation depth into the silicon can becontrolled, allowing three-dimensional control of the dopantconcentrations introduced by ion implantation. The dopant concentrationscontrol the electrical properties of the transistors, and hence theperformance of the IC's. A number of different electrically activedopant materials are used, including As, B, P, In, Sb, Be, and Ga. Manyof these materials can be obtained in gaseous chemical form, for exampleas AsH₃, PH₃, BF₃, PH₃, and SbF₅. The ion implanter is a manufacturingtool which ionizes the dopant-containing feed materials, extracts thedopant ions of interest, accelerates the dopant ion to the desiredenergy, filters away undesired ionic species, and then transports thedopant ion of interest to the wafer. Thus, the following variables mustbe controlled in order to achieved the desired implantation profile fora given implantation process:

Dopant feed material (e.g., BF₃ gas)

Dopant ion (e.g., B+)

Ion energy (e.g., 5 keV)

Chemical purity of the ion beam (e.g., <1% contaminants)

Energy purity of the ion beam (e.g., <2% FWHM).

An area of great importance in the technology of ion implantation is theion source. FIG. 1 shows the “standard” technology for commercial ionsources, namely the “Enhanced Bernas” ion source. This type of source iscommonly used in high current, high energy, and medium current ionimplanters. The ion source a is mounted to the vacuum system of the ionimplanter through a mounting flange b which also accommodates vacuumfeedthroughs for cooling water, thermocouples, dopant gas feed, N₂cooling gas, and power. The dopant gas feed c feeds gas into the arcchamber d in which the gas is ionized. Also provided are dual vaporizerovens e, f in which solid feed materials such as As, Sb₂O₃, and P may bevaporized. The ovens, gas feed, and cooling lines are contained within acooled machined aluminum block g. The water cooling is required to limitthe temperature excursion of the aluminum block g while the vaporizers,which operate between 100 C. and 800 C., are active, and also tocounteract radiative heating by the arc chamber d when the source isactive. The arc chamber d is mounted to, but in poor thermal contactwith, the aluminum block g. The ion source a is an arc discharge source,which means that it operates by sustaining a continuous arc dischargebetween an immersed hot-filament cathode h and the internal walls of thearc chamber d. Since this arc can typically dissipate in excess of 300W, and since the arc chamber d cools only through radiation, the arcchamber can reach a temperature in excess of 800 C. during operation.

The gas introduced to arc chamber d is ionized through electron impactwith the electron current, or arc, discharged between the cathode h andthe arc chamber d. To increase ionization efficiency, a uniform magneticfield i is established along the axis joining the cathode h and ananticathode j by external Helmholz coils, to provide confinement of thearc electrons. An anticathode j (located within the arc chamber d but atthe end opposite the cathode h) is typically held at the same electricpotential as the cathode h, and serves to reflect the arc electronsconfined by the magnetic field i back toward the cathode h and backagain repeatedly. The trajectory of the thus-confined electrons ishelical, resulting in a cylindrical plasma column between the cathode hand anticathode j. The plasma density within the plasma column istypically high, on the order of 10¹² per cubic centimeter; this enablesfurther ionizations of the neutral and ionized components within theplasma column by charge-exchange interactions, and also allows for thethe production of a high current density of extracted ions. The ionsource a is held at a potential above ground (i.e., the silicon waferpotential) equal to the accelerating voltage V_(a) of the ion implanter:the energy of the ions E as they impact the wafer substrate is given byE=qV_(a), where q is the electric charge per ion.

The cathode h is typically a hot filament or indirectly-heated cathode,which thermionically emits electrons when heated by an external powersupply. It and the anticathode are typically held at a voltage V_(c)between 60V and 150V below the potential of the ion source V_(a). Highdischarge currents D can be obtained by this approach, up to 7 A. Oncean arc discharge plasma is initiated, the plasma develops a sheathadjacent to the surface of the cathode h (since the cathode h isimmersed within the arc chamber and is thus in contact with theresulting plasma). This sheath provides a high electric field toefficiently extract the thermionic electron current for the arc; highdischarge currents can be obtained by this method.

The discharge power P dissipated in the arc chamber is P=DV_(c), orhundreds of watts. In addition to the heat dissipated by the arc, thehot cathode h also radiates power to the arc chamber d walls. Thus, thearc chamber d provides a high temperature environment for the dopantplasma, which also boosts ionization efficiency relative to a coldenvironment by increasing the gas pressure within the arc chamber d, andby preventing substantial condensation of dopant material on the hotchamber walls.

If the solid source vaporizer ovens e or f are used, the vaporizedmaterial feeds into the arc chamber d through vaporizer feeds k and l,and into plenums m and n. The plenums serve to diffuse the vaporizedmaterial into the arc chamber d, and are at about the same temperatureas the arc chamber d. Radiative thermal loading of the vaporizers by thearc chamber also typically prevents the vaporizers from providing astable temperature environment for the solid feed materials containedtherein below about 100 C. Thus, only solid dopant feed materials thatboth vaporize at temperatures >100 C. and decompose at temperatures >800C. can be vaporized and introduced by this method.

A very significant problem which currently exists in the ionimplantation of semiconductors is the limitation of ion implantationtechnology to effectively implant dopant species at low (e.g., sub-keV)energies. One critically important application which utilizes low-energydopant beams is the formation of shallow transistor junctions in CMOSmanufacturing. As transistors shrink in size to accommodate theincorporation of more transistors per IC, the transistors must be formedcloser to the silicon surface. This requires reducing the velocity, andhence the energy, of the implanted ions. The most critical need in thisregard is the implantation of low-energy boron, a p-type dopant. Sinceboron atoms have low mass, at a given energy they penetrate deeper intothe silicon than other p-type dopants, and must therefore be implantedat lower energies. Ion implanters are inefficient at transportinglow-energy ion beams due to the space charge within the ion beam causingthe beam profile to grow larger (beam blow-up) than the implanter'stransport optics, resulting in beam loss through vignetting. Inaddition, known ion sources rely on the application of a strong magneticfield in the source region. Since this magnetic field also exists in thebeam extraction region of the implanter, it deflects the low-energy beamand substantially degrades the emittance properties of the beam, furtherreducing beam transmission through the implanter. For example, at 500 eVtransport energy, many ion implanters currently in use cannot transportenough boron beam current to be useful in manufacturing; i.e., the waferthroughput is too low.

Recently, a new enabling technology has been pursued to solve theproblem of low-energy boron implantation: molecular beam ionimplantation. Instead of implanting an ion current I of atomic B+ions atan energy E, a decaborane molecular ion, B₁₀H_(x) ⁺, is implanted at anenergy 10×E and an ion current of 0.10×I. The resulting implantationdepth and dopant concentration (dose) of the two methods have been shownto be equivalent, but the decaborane implantation technique hassignificant advantages. Since the transport energy of the decaborane ionis ten times that of the dose-equivalent boron ion, and the ion currentis one-tenth that of the boron current, the space charge forcesresponsible for beam blowup and the resulting beam loss are much reducedrelative to monatomic boron implantation.

While BF₃ gas can be used by conventional ion sources to generateB⁺ions, decaborane (B₁₀H₁₄) must be used to generate the decaborane ionB₁₀H_(x) ⁺. Decaborane is a solid material which has a significant vaporpressure, on the order of 1 Torr at 20 C., melts at 100 C., anddecomposes at 350 C. It must therefore be vaporized below 100 C., andoperate in an ion source whose local environment (walls of the arcchamber and components contained within the arc chamber) are below 350C. In addition, since the B₁₀H₁₄ molecule is so large, it can easilydisassociate (fragment) into smaller components, such as elemental boronor diborane (B₂H₆), when subject to charge-exchange interactions withina dense plasma. Therefore, in order to preserve the B₁₀H_(x) ⁺ ion, theplasma density in the ion source must be low. Also, the vaporizers ofcurrent ion sources cannot operate reliably at the low temperaturesrequired for decaborane. This is due to radiative heating from the hotion source to the vaporizer causing thermal instability, and the factthat the vaporizer feed lines k, l easily become clogged with decomposedvapor as the decaborane vapor interacts with their hot surfaces. Hence,the prior art of implanter ion sources is incompatible with decaboraneion implantation.

SUMMARY OF THE INVENTION

The present invention provides an improved means for efficiently.

Vaporizing decaborane;

Delivering a controlled flow of vaporized decaborane into the ionsource;

Ionizing the decaborane into a large fraction of B₁₀H_(x) ⁺;

Preventing thermal dissociation of decaborane;

Limiting charge-exchange induced fragmentation of B₁₀H_(x) ⁺;

Operating the ion source without the use of an applied magnetic field,which improves the emittance properties of the beam.

Uses a novel approach to produce electron impact ionizations without theuse of an arc discharge, by incorporation of an externally generatedelectron beam which passes through the ionization chamber.

In addition, the present invention is compatible with current ionimplantation technology, such that the ion source can be retrofittedinto the existing fleet of ion implanters currently used in themanufacture of semiconductor devices.

DESCRIPTION OF THE DRAWINGS

These and other advantages will be readily understood with reference tothe following specification and attached drawing wherein:

FIG. 1 is a diagrammatic view of a prior art ion source known as anenhanced Bernas ion source.

FIG. 2 is a side-elevational view in cross-section of an ion source inaccordance with the present invention.

FIG. 3 is an elevational view in cross-section of an alternativeembodiment of a vaporizer for use with the ion source in accordance withthe present invention.

FIG. 4 is an elevational view of a portion of the ionization chamber,shown with the exit aperture removed, illustrating the configuration ofthe electron source within the ionization chamber.

FIG. 5 is a top view of the ionization chamber shown with the top wallremoved, illustrating the electron exit channel and its proximity to theextraction aperture.

FIG. 6 is a front elevational view of the ion source illustrated in FIG.2, illustrating the extraction aperture.

FIG. 7 is a perpective view of an exemplary embodiment of a cathodearrangement for use with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

An ion source for the generation of ions for use in ion implantation isdescribed that may be configured to be compatible with the designs ofcommercial ion implanters commonly in use in semiconductor manufacturingand in the surface modification of materials. Such an ion source can bereadily retrofitted into commercial ion planters as a direct replacementfor their commercial ion source.

The ion source described herein is composed of a i) vaporizer, ii)vaporizer valve, iii) gas feed, iv) ionization chamber, v) electron gun,vi) source shield, and vii) exit aperture. In particular, means forintroducing gaseous feed material into an ionization chamber, means forvaporizing and introducing solid feed materials into the ionizationchamber, means for ionizing the introduced gaseous feed materials withinthe ionization chamber and means for extracting the ions thus producedfrom an exit aperture adjacent to the ionization chamber. In addition,means for accelerating and focusing the exiting ions is described. Thevaporizer, vaporizer valve, gas feed, ionization chamber, electron gun,source shield, and exit aperture are all integrated into a singleassembly collectively identified the ion source.

Vaporizer: the vaporizer is designed for vaporizing solid materials,such as decaborane (B₁₀H₁₄) and TMI (Trimethyl Indium), which haverelatively high vapor pressures at room temperature, and thus vaporizeat temperatures below 100 C. For example, solid decaborane has a vaporpressure of about 1 Torr at 20 C. Most other implant species currentlyof interest in the ion implantation of semiconductors, such as As, P,Sb, B, C, Ar, N, Si, and Ge are available in gaseous forms (e.g.,A_(s)H₃, PH₃, SbF₅, BF₃, CO₂, Ar, N₂, SiF₄, and GeF₄ gases). However,B₁₀ and In are not, but can be vaporized from decaborane and TMI. Thevaporizer is a machined aluminum block wherein resides a sealed cruciblecontaining the solid material to be vaporized, entirely surrounded by aclosed-circuit water bath, which is itself enclosed by the aluminumblock. The bath is held at a well-defined temperature by a closed-looptemperature control system linked to the vaporizer. The closed-looptemperature control system incorporates a PID (Proportional IntegralDifferential) controller which accepts a user-programmable temperaturesetpoint, and activates a resistive heater (which is mounted to a heaterplate in contact with the water bath) to reach and maintain it'ssetpoint temperature through a thermocouple readback circuit whichcompares the setpoint and readback values to determine the proper valueof current to pass through the resistive heater. To ensure goodtemperature stability, a water-cooled heat exchanger coil is immersed inthe water bath to continually remove heat from the bath, which reducesthe settling time of the temperature control system. The temperaturedifference between the physically separate heater plate and heatexchanger coil provides flow mixing of the water within the bath throughthe generation of convective currents. As an added mixing aid, arotating magnetic mixer paddle can be incorporated into the water bath.Such a temperature control system is stable from 20 C. to 100 C. Theflow of gas from the vaporizer to the ionization chamber is determinedby the vaporizer temperature, such that at higher temperatures, higherflow rates are achieved.

An alternative embodiment of the operation of the vaporizer PIDtemperature controller is described as follows. In order to establish amore repeatable and stable flow, the vaporizer PID temperaturecontroller receives the output of an ionization-type pressure gaugewhich is typically located in the source housing of commercial ionimplanters to monitor the sub-atmospheric pressure in the sourcehousing. Since the ionization gauge output is proportional to the gasflow into the ion source, its output can provide an input to the PIDtemperature controller. The PID temperature controller can subsequentlyraise or diminish the vaporizer temperature, thus increasing ordecreasing gas flow into the source, until the desired gauge pressure isattained. Thus, two complementary operating modes of the PID temperaturecontroller are defined: temperature-based, and pressure-based. These twoapproaches can be combined so that short-term stability of the flow rateis accomplished by temperature programming alone, while long-termstability of the flow rate is accomplished by adjusting the vaporizertemperature to meet a pressure setpoint. The advantage of such anapproach is that, as the solid material in the vaporizer crucible isconsumed, the vaporizer temperature can be increased to compensate forthe smaller flow rates realized by the reduced surface area of thematerial presented to the vaporizer.

In an alternate embodiment of the above-described vaporizer, a waterbath is not used. Rather, the crucible is integral to the machined bodyof the vaporizer, and heating and cooling elements are embedded into thealuminum wall of the vaporizer. The heating element is a resistive orohmic heater, and the cooling element is a thermoelectric (TE) cooler.The vaporizer is also encased in thermal insulation to prevent heat lossto the ambient, since the desired vaporizer temperature is typicallyabove room temperature. In this embodiment, the heating/cooling elementsdirectly determine the temperature of the walls of the vaporizer, andhence the material within the crucible, since said material is in directcontact with the walls of the vaporizer which is machined of a singlepiece of aluminum. The same PID temperature controller can be used as inthe embodiments described above.

Vaporizer valve: In the above described vaporizer embodiments, thevaporized material enters the adjacent ionization chamber of the ionsource through an aperture, which is coupled to a thin, metal-sealedgate valve placed between the vaporizer and ionization chamber. The gatevalve serves to separate the vaporizer from the ionization chamber, sothat no vapor escapes the vaporizer when the valve is shut, but a short,high-conductance line-of-sight exists between the ionization chamber andvaporizer when the valve is open, thus allowing the vapors to enter theionization chamber. With the valve in the closed position, the vaporizermay also be removed from the ion source without releasing the vaporizermaterial contained in the crucible. The ion source may then be sealed byinstalling a blank flange in the position previously occupied by thevaporizer valve.

Gas feed: In order to run gaseous feed materials, ion implanterstypically use gas bottles which are coupled to a gas distributionsystem. The gases are fed to the ion source via metal gas feed lineswhich directly couple to the ion source through a sealed VCR or VCOfitting. In order to utilize these gases, the herein described ionsource likewise has a gas fitting which couples to the interior of theionization chamber.

Ionization chamber: The ionization chamber is where the neutral gas fedor vaporized into the source is ionized by electron impact. Theionization chamber is in intimate thermal and mechanical contact withthe vaporizer valve through a thermally conductive metal gasket, whichis likewise in intimate thermal contact with the vaporizer through athermally conductive, thin metal gasket. This provides temperaturecontrol of the ionization chamber through thermal contact with thevaporizer. This feature is important, since the plasma generated in theionization chamber can heat the walls of the chamber to temperatureswhich can cause decaborane or other low-temperature vaporized materialsto break down and disassociate. The ionization chamber is rectangular,made of a single piece of machined aluminum, molybdenum, graphite, orother suitable thermally conductive material. The ionization chamber isapproximately 3 inches tall by 2 inches wide by 2 inches deep; thechamber wall thickness is approximately 0.38 inch. Thus, the ionizationchamber has the appearance of a hollow, rectangular five-sided box. Thesixth side is occupied by the exit aperture, which we will describelater. The flow rate of the gas fed into the ionization chamber must besufficient to maintain proper feed gas pressure within the ionizationchamber. For most materials, including decaborane, a pressure between0.5 mtorr and 5 mtorr will yield good ionization efficiency. We notethat the ion gauge mounted in the source housing, typically used incommercial ion implanters to monitor source pressure, would read between1×10⁻⁵ Torr and 1×10⁻⁴ Torr. The flow rate from the vaporizer or gasfeed into the ionization chamber required to sustain this pressure wouldbe between 1 sccm and 5 sccm (standard cubic centimeters per minute).

Electron gun: In order to ionize the gases within the ionizationchamber, energetic electrons must be introduced into the ionizationchamber. In our invention, a high-current electron gun is mountedadjacent to one end of the ionization chamber, but external to thatchamber, such that a directed stream of energetic electrons is injectedinto the ionization chamber along the long axis of the rectangularchamber. The cathode of the electron gun is held at an electricpotential below the potential of the ionization chamber by a voltageequal to the desired energy of the electron beam. Two apertures areprovided in the ionization chamber wall to accommodate the electronbeam, one aperture for entrance of the beam and a second aperture forthe exit of the beam. After the electron beam exits the ionizationchamber, it is intercepted by a beam dump located just outside of theionization chamber. The electron beam is of a variable energy andcurrent to accommodate the specific ionization needs of the various feedmaterials introduced into the ionization chamber, and also the specificion currents required by the ion implant processes of the end-user. Inparticular, the electron gun provides an electron beam energyprogrammable between 20 eV and 1 keV. The lowest beam energies in thisenergy range would accommodate selective ionization of the gas belowcertain ionization threshold energies, limiting the different kinds ofend-product ions produced from the neutral gas species. An example wouldbe producing B₁₀H_(x) ⁺ ions without significant production of B₉H_(x)⁺, B₈H_(x) ⁺, and other lower-order boranes frequently contained in thedecaborane cracking pattern when higher electron impact energies areused. The highest beam energies in the energy range of the electron gunwould accommodate the formation of multiply-charged ions, for example,B⁺⁺ and B³⁺ from BF₃ feed gas. For the majority of ion production fromthe various feed gases used in semiconductor manufacturing, includingthe production of B₁₀H_(x) ⁺ from decaborane, an electron beam energybetween 50 eV and 150 eV would yield good results.

The electron beam also accommodates a range of injected electron beamcurrents between 1 mA and 5 A, in order to determine the ion currentextracted from the ion source. Control of electron current isaccomplished by a closed-loop electron gun controller which adjusts theelectron emitter temperature and the electron gun grid potential tomaintain the desired electron current setpoint. The electron emitter, orcathode, emits electrons by thermionic emission. It must thereforeoperate at elevated temperatures. To this end, the cathode may bedirectly heated (by passing an electric current through the cathodematerial), or indirectly heated. Cathode heating by electron bombardmentfrom a hot filament held behind the cathode is an indirect heatingtechnique well-practiced in the art. The cathode may be made oftungsten, tantalum, lanthanum hexaboride (LaB₆), or other refractoryconductive material. LaB₆ would offer a particular advantage, in that itemits copious currents of electrons at lower temperatures than tungstenor molybdenum.

The shape of the electron beam has a rectangular cross section;approximately 0.75 inch×0.25 inch as injected into the ionizationchamber. The shape of the injected electron beam is determined by theshapes of the grid and anode apertures in the electron gun, which areboth approximately 0.75 inch×0.25 inch, and also by the shape of thecathode or electron emitter, which is somewhat larger than the grid andanode apertures, approximately 0.9 inch×0.35 inch. The purpose ofgenerating a rectangular electron beam profile is to match the desiredion beam profile as extracted from the ion source, which is alsorectangular. The rectangular exit aperture from which the ion beam isextracted is approximately 2 inches tall by 0.5 inch wide; the electronbeam (and thus the ions produced by electron impact) would present aprofile to the exit aperture within the ionization chamber ofapproximately 2.5 inch×0.75 inch.

There is both an entrance and exit aperture for the electron beam, whichdeparts from the prior art. Prior art allows the energetic electronsproduced by an emitter internal to the ionization chamber to strike thewalls of the chamber; this forms the basis of an “arc discharge” ionsource, and also provides a substantial heat load which elevates thetemperature of the ionization chamber. In our invention, the ionizingelectrons (called energetic or “primary” electrons) pass through theionization chamber, substantially without intercepting the chamberwalls. However, the “secondary” electrons, or the low-energy electronsproduced by ionization of the feed gas, can strike the ionizationchamber walls. Since these are low energy electrons, they do not providea significant heat load to the ionization chamber. This feature allowsthe ionization chamber to be conductively cooled by the vaporizerwithout providing a large heat load on the vaporizer temperaturecontroller. To further contain the heat generated by the electron gunand the energetic electron beam, both the electron gun and the electronbeam dump are mounted on a water-cooled shield, called the sourceshield. This shield is cooled by low-resistivity, de-ionized watercommonly provided in commercial ion implanters.

Source Shield: The source shield is a water-cooled sheet metal assemblyon which the electron gun and the electron beam dump are mounted. Bymounting these two components to the water-cooled shield, the heat loadto the ionization chamber can be substantially reduced. The shieldprovides a mechanical framework for the thus-mounted components, and inaddition the shield and the mounted components can be held at anelectric potential different from the potential of the ionizationchamber and vaporizer by mounting the shield to the source onelectrically insulating standoffs. There are two embodiments of thesource shield: the first embodiment maximizes the conductance of the ionsource to the vacuum system of the ion implanter, and the secondembodiment minimizes that conductance. To clarify, in the firstembodiment the shield has two projections, one for mounting the electrongun, and the second for mounting the beam dump. The projections need beonly slightly larger in diameter than these two components to accomplishit's function of providing mechanical stability and coiling to thesecomponents. However, if the shield were rather of a rectangular orcylindrical design, it would shield the source assembly from the vacuumhousing it resides in within the implanter. The advantage of thisapproach would be to protect the implanter components from contaminationfrom the gases fed into the ion source during operation, reducingimplanter maintenance requirements. The second embodiment would alsoreduce the gas conductance from the source to the implanter vacuumsystem, increasing the pressure differential between the ionizationchamber and the implanter vacuum system. This feature would result in areduction in the gas flow into the ionization chamber required tomaintain the ionization chamber at a given feed gas pressure.

In an alternate embodiment of the invention, the electron beam dump isbiased to a negative potential relative to the ionization chamber, atapproximately the cathode potential, allowing for a “reflex geometry”whereby the primary electrons emitted by the electron gun are reflectedfrom the beam dump back into the ionization chamber and to the cathode,and back again repeatedly. An axial magnetic field may also beestablished along the direction of the electron beam by the introductionof a pair of Helmholtz coils external to the ion source, to provideconfinement of the primary electron beam as it is reflected back andforth between the cathode and beam dump. This feature also provides someconfinement for the ions, increasing the efficiency of creating certaindesired ion products, for example B+from BF₃, feed gas.

FIG. 2 shows in schematic the first embodiment of the ion source 1. Thevaporizer 2 is attached to the vaporizer 3 through a annular metalgasket 4. The vaporizer valve 3 is likewise attached to the ionizationchamber 5 by a second annular metal gasket 6. This ensures good thermalconduction between the vaporizer, vaporizer valve, and ionizationchamber 5 through intimate via thermally conductive elements. Amountingflange 7 attached to the ionization chamber 5 allows mounting of the ionsource 1 to the vacuum housing of an ion implanter, and containselectrical feedthroughs (not shown) to power the ion source, andwater-cooling feedthroughs 8, 9 to cool the ion source. In the preferredembodiment of the invention, water feedthroughs 8, 9 circulate waterthrough the source shield 10 to cool the source shield 10 and cool theattached components, the beam dump 11 and electron gun 12. The exitaperture 13 is mounted to the ionization chamber 5 face by metal screws(not shown). Thermal conduction of the exit aperture 13 to theionization chamber 5 is aided by an annular seal 14 which can be madefrom metal or a thermally conductive polymer.

When the vaporizer valve 3 is in the open position, vaporized gases fromthe vaporizer 2 can flow through the vaporizer valve 3 to inlet channel15 into the open volume of the ionization chamber 16. These gases areionized by interaction with the electron beam transported from theelectron gun 12 to the beam dump 11. The ions can then exit the ionsource from the exit aperture 13, where they are collected andtransported by the ion optics of the ion implanter.

The vaporizer 2 is made of machined aluminum, and houses a water bath 17which surrounds a crucible 18, wherein resides solid feed materials suchas decaborane 19. The water bath 17 is heated by a resistive heaterplate 20 and cooled by a heat exchanger coil 21 to keep the water bathat the desired temperature. The heat exchanger coil 21 is cooled byde-ionized water provided by water inlet 22 and water outlet 23.Although the temperature difference between the heating and coolingelements provides convective mixing of the water, a magnetic paddlestirrer 24 continuously stirs the water bath 17 while the vaporizer isin operation. A thermocouple 25 continually monitors the temperature ofthe crucible 18 to provide temperature readback for a PID vaporizertemperature controller (not shown). The ionization chamber 5 is made ofaluminum, graphite, or molybdenum, and operates near the temperature ofthe vaporizer 2 through thermal conduction. In addition tolow-temperature vaporized solids, the ion source can receive gasesthrough gas feed 26, which feeds directly into the open volume of theionization chamber 16 by an inlet channel 27. Typical feed gasesprovided for the ion implantation of semiconductors are AsH₃, PH₃, SbF₅,BF₃, CO₂, Ar, N₂, SiF₄, and GeF₄. When the gas feed 26 is used to inputfeed gases, the vaporizer valve 3 is closed.

The vaporizer can be demounted from the ion source 1 by closing thevaporizer valve 3 and removing the seal 6. This is useful for rechargingthe solid feed material in the crucible 18, and for maintenanceactivities.

Referring now to FIG. 3, we describe an alternative embodiment of theion source, whereby the vaporizer 2 is of a different design. However,the rest of the ion source 2 is the same as in FIG. 2 previouslydescribed. In the second embodiment of the vaporizer 28, there is nowater bath or water-fed heat exchanger. Instead, the volume previouslyoccupied by the water bath 17 is occupied by the machined aluminum body29 of the vaporizer 28. A resistive heater plate 20 is in direct contactwith the vaporizer body 29 to heat the body 29, and a thermoelectric(TE) cooler 30 is in direct contact with the vaporizer body 29 toprovide cooling. A thermally insulating sleeve 31 surrounds thevaporizer 28 to thermally insulate the vaporizer from ambienttemperature. If desired, several heater plates 20 and TE coolers 30 canbe distributed within the vaporizer body 29 to provide more heating andcooling power, and also to provide a more spatially uniform temperatureto the crucible.

Referring now to FIG. 4, we further describe the operation of the ionsource. An electron beam 32 is emitted from the cathode 33 and focusedby the electron optics 34 to form a wide beam. The electron beam isasymmetric, in that it is wider perpendicular to the ion beam axis thanit is along that axis. FIG. 4 illustrates the geometry of the ion sourcewith the exit aperture removed; the ion beam axis points out of theplane of the paper. The distribution of ions created by neutral gasinteraction with the electron beam roughly corresponds to the profile ofthe electron beam. Since the exit aperture 13 shown in FIG. 6 is a wide,rectangular aperture, the distribution of ions created adjacent to theaperture 13 should be uniform. Also, in the ionization of decaborane andother large molecules, it is important to maintain a low plasma densityin the ion source. This limits the charge-exchange interactions betweenthe ions which can cause loss of the ions of interest. Since the ionsare generated in a widely distributed electron beam, this will reducethe local plasma density relative to other conventional ion sourcesknown in the art. The electron beam passes through a rectangularentrance channel 35 in the ionization chamber and interacts with theneutral gas within the open volume 16. It then passes through arectangular exit channel 36 in the ionization chamber and is interceptedby the beam dump 11, which is mounted onto the water-cooled sourceshield 10. Since the heat load generated by the hot cathode 33 and theheat load generated by impact of the electron beam 32 with the beam dump11 is substantial, these elements are kept outside of the ionizationchamber open volume 16 where they cannot cause dissociation of theneutral gas molecules and ions. In addition, the only heat load fromthese elements to the ionization chamber is through radiation, so theionization chamber can be effectively cooled by thermal contact with thevaporizer 2. Thus, the ionization chamber walls can be maintained at atemperature below the dissociation temperature of the neutral gasmolecules and ions. For decaborane, this dissociation temperature isabout 350 C.

FIG. 5 shows a top view of the exit channel 36 in the ionization chamber5, and it's proximity to the exit aperture 13. Since the ions areremoved from the ionization chamber by penetration of an electrostaticextraction field outside of the ion source 1 through the exit aperture13, the electron beam 32 and exit channel 36 are close to the exitaperture 13, allowing for more efficient removal of ions. Although theelectron beam 32 may not fully retain it's rectangular profile due toscattering, and also due to space charge forces within the electron beam32, the exit channel 36 can still be sized to allow passage of theelectron beam without significant interception by the ionization chamber5.

Referring now to FIG. 6, the figure shows the exit aperture 13 with theaxis of the ion beam directed normal to the plane of the paper. Thedimensions of the exit aperture plate conform to the dimensions of theionization chamber 5, approximately 3 inches tall×2 inches wide. Theexit aperture contains an opening 37 which is approximately 2 inchestall×0.5 inch wide, and has a bevel 38 to reduce strong electric fieldsat its edges.

Referring now to FIG. 7, the figure shows the shape of the cathode 33,or electron emitter. It's dimensions are roughly 0.9 inch long×0.35 inchwide×0.125 inch thick. It can be directly heated by passing an electriccurrent through it, or it can be indirectly heated. We show thepreferred embodiment, that of an indirectly-heated cathode. An electriccurrent flows through a filament 39 through leads 40, heating thefilament 39 to emit thermionic electrons 41. By biasing the filament 39to a voltage several hundred volts below the potential of the cathode33, the thermionic electrons 41 will heat the cathode 33 by energeticelectron bombardment, as is known in the art.

What is claimed and desired to be secured by a Letters Patent of the United States is:
 1. An ion source comprising a mounting flange for mounting to an ion source housing, an ionization chamber supported from an inner surface of the mounting flange and a vaporizer removably mounted directly on an exterior of the flange, and a conduit extending from the vaporizer through the flange to the ionization chamber.
 2. The ion source of claim 1 in which the vaporizer includes a gate valve associated with the conduit, the valve for sealing the vaporizer before it is removed from the mounting flange.
 3. An ion source for providing a source of ions to an ion implanter, the ion source comprising: an ionization chamber, said ionization chamber including a first wall having a vapor inlet aperture and a spaced apart generally parallel second wall having an exit aperture for enabling ionized vapor to exit from said ionization chamber in a form of an ion beam, said ionization chamber including third and fourth spaced apart walls generally perpendicular to said first and second walls for closing said ionization chamber, said third and fourth walls formed with aligned apertures defining electron beam apertures; a source of electrons, disposed outside of said ionization chamber adjacent one of said electron beam apertures and configured to direct a beam of electrons into said ionization chamber along a path defined by said electron beam apertures; a beam dump disposed adjacent the other of said electron beam apertures, outside of said ionization chamber; a vapor source for supplying gas to the ionization chamber; and a conduit coupled between said vapor source and said ionization chamber.
 4. The ion source as recited in claim 3, wherein said source of electrons is an electron gun.
 5. The ion source as recited in claim 3, wherein the path of said ion beam is generally transverse to the path of said beam of electrons.
 6. The ion source as recited in claim 3, wherein the vapor source is an external source of feed gas.
 7. The ion source recited in claim 6, wherein the feed gas is selected from a group of AsH₃, PH₃, SbF₅, BF₃, CO₂, Ar, N₂, SiF₄, GeF₄.
 8. The ion source as recited in claim 3, wherein said vapor source includes a vaporizer for vaporizing a solid feed material.
 9. The ion source as recited in claim 8, wherein said solid feed material is decaborane.
 10. The ion source as recited in claim 8, further including a vaporizer valve between said vaporizer and said conduit.
 11. The ion source as recited in claim 10, wherein said vaporizer and said ionization chamber are in thermal conduction. 